Ohio astronauts Mark Brown and Ronald Parise know what it's like to sit in the cabin of the space shuttle Columbia as the ship plunges through the Earth's atmosphere, traveling 18 times faster than the speed of sound.

They have felt the extreme heat of re-entry through the faceplates of their spacesuits, and they have watched pieces of the shuttle's heat shield fly past the window as the ship's exterior glowed a brilliant orange.

"You see behind you the fiery contrail," Brown said Saturday. "You're very much aware of what's going on."

More than anyone, Brown and Parise know the risks associated with a shuttle mission.

And yet they were as shocked as the rest of the nation Saturday as they watched Columbia break apart during re-entry.

Parise, who flew on Columbia in 1990, said he turned on the television Saturday morning expecting to see the shuttle and its crew safely on the ground in Florida. Instead, he watched video images of the shuttle's remains falling to Earth from 200,000 feet.

"I just thought, `Oh, my God,'" said Parise, originally of Warren, Ohio. "I didn't know what happened, but it looked bad."

Aside from his experience on Columbia, the disaster was personal for Parise because he had met the shuttle crew who died Saturday. His Maryland-based company, Computer Sciences Corp., had a communications experiment on board Columbia and he had worked with the astronauts on several occasions.

"Anybody who has gotten to that point in the astronaut corps is obviously very competent and personable," Parise said. "They wouldn't have gotten that far if they weren't.

"They're all very nice people."

Parise, a payload specialist, described Columbia as "a good ship" and said it had performed well during his first mission in 1990.

Both he and Brown said Columbia was the sturdiest of the shuttle fleet. Built in 1979, Columbia weighed 7,800 pounds more than any other shuttle because it was equipped with a thicker heat shield and more structural features.

The extra bulk was built in because, as the first shuttle, NASA engineers wanted to be certain it could withstand the stresses of liftoff and re-entry. Sensors on early Columbia flights showed that shuttles built later did not need to be as strong.

"The differences in the orbiters are fairly profound," said Brown, a Dayton resident who served as a mission specialist on Columbia in 1989. "This is really a puzzle."

Although re-entry is considered one of the most dangerous times in a shuttle mission, both Brown and Parise said the ship is on autopilot and the ride is smooth because the air is so thin.

"That phase of the flight is fairly benign," Parise said.

Outside, however, friction with the atmosphere is extreme and the shuttle is exposed to temperatures of 1,800 degrees. It gets so hot, that astronauts sometimes see glowing tiles from the heat shield break off from the ship and burn up as they fly past the window.

Brown said TV footage seemed to show that a panel from the top of the shuttle broke off shortly before the shuttle was destroyed. But he would not speculate about what might have caused the disaster.

He said the most immediate concern is for the families of the astronauts.

"My heart goes out to the families," Brown said, recalling what his wife and two children went through when he was in space. "It's so hard being a family member during a shuttle mission."

Although they said great care must be taken to find the cause of the tragedy, Brown and Parise said they are optimistic it will not jeopardize the future of the shuttle program or of space exploration.